Production Of Nano-Structures

ABSTRACT

A process for the production of nano-structures is presented, involving providing a graphite flake comprising graphene layers; intercalating the graphite flake to form a graphite intercalation compound exhibiting Stage I, II or III intercalation; and exfoliating the graphite intercalation compound by exposing it to a temperature between about 1600° C. and about 2400° C. such that a plurality of individual graphene layers are separated from the graphite intercalation compound.

RELATED APPLICATION

The present application claims priority under 35 U.S.C. §120 fromcopending and commonly assigned U.S. patent application Ser. No.11/422,914, entitled Production of Nano-Structures, filed in the name ofRobert A. Mercuri on Jun. 8, 2006, the disclosure of which isincorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to the production of nano-structures,such as nano-tubes, Buckminster fullerenes (commonly referred to as“buckyballs”), and nano-scale plates. More particularly, the disclosurerelates to the production of nano-structures in a process capable of theefficient production of commercial quantities of such nano-structures,using natural graphite starting materials.

BACKGROUND OF THE DISCLOSURE

Nano-structures, especially nano-tubes and buckyballs, have been thesubject of extensive research; they have remarkable tensile strength andexhibit varying electrical properties, such as superconducting,insulating, semi-conducting or conducting, depending on their helicity,and are thus utilizable as nanoscale wires and electrical components.The electrical conductivity is as high or higher than copper, thermalconductivity as high as diamond, and the tensile strength of thesestructures can be 100 times greater than steel, leading to structuresthat have uses in space, and that are believed to have applications asdiverse as the formation of field-effect transistors and nano-motors.Indeed, there are those who believe nano-tubes and other nano-scalestructures can be the solution to the hydrogen storage issues bedevilingthe nascent hydrogen fuel cell industry, since hydrogen can be adsorbedon their surface.

When referring to nano-structures, what is meant is a structure whichis, on average, no greater than about 1000 nanometers (nm), e.g., nogreater than about one micron, in at least one dimension. Therefore, inthe case of a nano-scale plate, the thickness (or through-planedimension) of the plate should be no greater than about 1000 nm, whilethe plane of the plate can be more than one millimeter across; such anano-plate would be said to have an aspect ratio (the ratio of themajor, or in-plane, dimension to the minor, or through-plane, dimension)that is extremely high. In the case of a nano-tube, the average internaldiameter of the tube should be no greater than about 1000 nm (thus, witha length of up to a millimeter (mm), the aspect ratio of nano-tubes isalso extremely large); in the case of a buckyball, the diameter of thebuckyball, such as the truncated icosahedron (the shape of a 60-carbonbuckyball), should be no greater than about 1000 nm. A minor dimensionof the nano-structure (for instance, the thickness of a nano-scale plateor the internal diameter of a nano-tube), should preferably be nogreater than about 250 nm, most preferably no greater than about 20 nm.

Unfortunately, the production of commercial-scale quantities ofnano-structures is expensive, laborious and time-consuming, to theextent that doing so is not considered feasible. Production processescurrently employed include high pressure carbon monoxide conversion(HiPCO), pulsed-laser vaporization (PLV), chemical vapor deposition(CVD) and carbon arc synthesis (CA). None of these processes isconsidered adequate in the long term.

Natural graphite is formed of layered planes of hexagonal arrays ornetworks of carbon atoms, with extremely strong bonds within the layers,and relatively weak bonding between the layers. The carbon atoms in eachlayer plane (generally referred to as basal planes or graphene layers)are arranged hexagonally such that each carbon atom is covalently bondedto three other carbon atoms, leading to high intra-layer strength.However, the bonds between the layers are weak van der Waals forces(which are less than about 0.4% of the strength of the covalent bonds inthe layer plane). Accordingly, because these inter-layer bonds are soweak as compared to the covalent intra-layer bonds, the spacing betweenlayers of the graphite particles can be chemically or electrochemicallytreated so as to be opened up to provide a substantially expandedparticle while maintaining the planar dimensions of the graphene layers.

It is this characteristic of natural graphite which is exploited in theproduction of sheets of compressed particles of exfoliated graphite(often referred to in the relevant industry as “flexible graphite”),which is used in the production of, inter alia, gasket materials, fuelcell components, electronic thermal management articles and devices,etc. As taught by Shane et al. in U.S. Pat. No. 3,404,061, naturalgraphite flakes can be intercalated by dispersing the flakes in asolution of a mixture of nitric and sulfuric acids. After intercalation,the flakes can be drained and washed, and are then exposed totemperatures, such as from about 700° C. to about 1000° C., with a hightemperature of about 1200° C., which causes the flakes to expand in anaccordion-like fashion in the direction perpendicular to the planes ofthe particle, by an amount that can be greater than 80 times, and asmuch as about 1000 times or greater, to form what are commonly called“worms.” These worms can then be formed in to sheets, even without thepresence of binders, which can be formed, cut, molded and otherwisedeformed.

Additional processes for the production of these sheets of compressedparticles of exfoliated graphite are taught by, for instance, Mercuri etal. in U.S. Pat. No. 6,432,336, Kaschak et al. in InternationalPublication No. WO 2004/108997, and Smalc et al. in U.S. Pat. No.6,982,874. The unique directional properties of natural graphite (whilegraphite is commonly referred to as anisotropic, from a crystallographicstandpoint, graphite should more properly be referred to as orthotropicor exhibiting transverse isotropy; in the plane of sheet, it isisotropic in two directions along the plane) provide sheets ofcompressed particles of exfoliated graphite having directionalelectrical and thermal characteristics, where conductivity issubstantially higher along the plane of the sheet as opposed to throughthe sheet, is leveraged in the production of thermal management articlesand fuel cell components.

The intercalation process described above functions to insert a volatilespecies between the layer planes of the graphite flake which, whenexposed to high temperatures, rapidly volatilizes, causes separation ofthe layers and, consequently, exfoliation. Typical intercalation ofgraphite for the production of sheets of compressed particles ofexfoliated graphite is Stage VII or greater Stage value. The Stage Indexis a measure of the average number of graphene layers between each“gallery” (the space between graphene layers in which the chemicalintercalant is inserted), rounded to the nearest whole number.Therefore, in Stage VII intercalation, there are, on average, less than7.5 graphene layers between each gallery. In Stage VIII intercalation,there are, on average, at least 7.5 graphene layers between eachgallery.

The Stage Index of an intercalated graphite flake can be determinedempirically by x-ray diffraction to measure the “c” lattice spacing (thespacing between any three graphene layers), where a spacing of 6.708indicates (Å) represents a non-intercalated graphite flake and over 8 Åindicates an intercalated flake with Stage I intercalation (on average,only one graphene layer separating each gallery, or as completeintercalation as possible).

Processes for preparing lower intercalation Stages (more specifically,Stage III and lower) are known. For instance, Kaschak et al.(International Publication No. WO 2004/108997) described a process forpreparing Stage V (i.e., intercalation between, on average, every fifthgraphene layer) or lower intercalation using supercritical fluids. Othersystems for preparing intercalated graphite flakes having Stage III orhigher degree intercalation (that is, intercalation to Stage I, II orIII) using methanol, phosphoric acid, sulfuric acid, or simply water,combined with nitric acid in various combinations, are known, for both“normal” or “spontaneous” intercalation and electrochemicalintercalation.

For instance, an admixture of up to 15% water in nitric acid can provideStage III or II spontaneous intercalation and Stage I electrochemicalintercalation; for methanol and phosphoric acid, an admixture of up to25% in nitric acid can provide Stage II spontaneous intercalation andStage I electrochemical intercalation. The chemical or electrochemicalpotential of the intercalant critically effects the thermodynamics ofthe process, where higher potential leads to a lower stage number (i.e.,a greater degree of intercalation), while kinetic effects such as timeand temperature combine to define processes which can be of commercialimportance.

What is desired, therefore, is a process for preparing nano-structuresin a cost-effective and commercially feasible manner. The desiredprocess will enable the production of nano-structures, whethernano-tubes, buckyballs or nano-plates, in quantities sufficient forindustry-scale uses without the requirement of exotic equipment, unusualraw materials or extreme process parameters.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a process for producing nano-scalestructures using Stage III or lower graphite intercalation compounds(GICs)(that is, GICs intercalation to Stage I, II or III). GICs aregraphite flakes which have been treated with an intercalant underconditions such that a volatile compound is inserted between layerplanes of the graphite flake. The intercalation can be spontaneousintercalation or electrochemical intercalation.

The GICs are then exposed to sufficient heat to cause expansion of theintercalated graphite flakes under conditions which cause at least someof the individual graphene layers to separate and thus formnano-structures. In certain embodiments, the GICs are fed into a gasplasma or directly into an arc in a protective environment, such as inan inert gas, or a conditioning gas, such as hydrogen, which can bond toand thus protect active sites. For expansion, high heat flux at theGIC's and reduced pressure both are consistent with energy and inert gasconservation, and provide expansion rather than de-intercalation. Inthis way, nano-structures are prepared in commercial quantities fromnatural graphite flakes which have been intercalated so as to form StageIII or lower GICs. At lower heat flux, the GIC is exposed to lowertemperatures, which can cause the intercalant to be removed withrelatively little disruption (i.e., exfoliation) in the direction of thecrystal perpendicular to the graphene layer.

Therefore, an aspect of the present disclosure relates to a process forproducing nano-scale structures, such as nano-tubes, Buckminsterfullerenes, nano-plates and the like.

Another aspect of the disclosure relates to a process for producingnano-scale structures, which is capable of producing commercialquantities of nano-scale structures.

Yet another aspect of the present disclosure relates to an efficientprocess for producing nano-scale structures.

Still another aspect of the present disclosure relates to a process forproducing nano-scale structures from natural graphite flakes.

Another aspect of the present disclosure relates to a process forproducing nano-scale structures from natural graphite flakes in aprocess which does not require the use of exotic equipment or extremeprocess parameters.

These aspects of embodiments of the disclosure, and others which willbecome apparent to the artisan upon review of the following descriptioncan be accomplished by providing a process for the production ofnano-structures, which includes providing a graphite flake comprisinggraphene layers; intercalating the graphite flake to form a graphiteintercalation compound exhibiting Stage I, II or III intercalation; andexfoliating the graphite intercalation compound by exposing it to atemperature between about 1600° C. and about 2400° C., such that aplurality of individual graphene layers are separated from the graphiteintercalation compound. In some embodiments, at least some of theplurality of individual graphene layers spontaneously form a nano-tubeor a Buckminster fullerene. The graphite intercalation compound is, incertain embodiments, brought from a temperature at which it is stable toa temperature between about 1600° C. and about 2400° C. in less than 1second, more preferably less than 0.5 second, and most preferably lessthan about 0.1 second.

In some embodiments, the graphite intercalation compound is exfoliatedby passing it through a reaction zone which includes a region where thetemperature is at least about 2600° C., provided that the graphiteintercalation compound does not reach a temperature of 2500° C. orhigher. In certain embodiments, the reaction zone is at an averagetemperature of at least about 1500° C.

The graphite flake is preferably intercalated with an intercalantcomprising formic acid, acetic acid, water, or combinations thereof, andthe graphite intercalation compound is exposed to a supercritical fluidprior to exfoliation. Exfoliation preferably involves suddenly exposingthe graphite intercalation compound to a temperature significantly abovethe decomposition temperature of the GICs. Most preferably thetemperature of exfoliation should exceed 1500° C., and temperaturesexceeding about 2500° C. or even plasma temperatures as great as about10000° C. or higher can be employed in the process of the presentdisclosure. As noted, residence time at these temperatures should beless than 1 second. As noted, it is most preferred that exfoliation takeplace in an inert or protective environment in order to avoid oxidationof the graphite.

In an advantageous embodiment, exfoliation is accomplished by feedingthe graphite intercalation compound into a reaction zone having an inertgas plasma or directly into an arc, especially in a reducing gasenvironment, such as a hydrogen environment.

The graphite intercalation compound can be exfoliated by beingcontinuously extruded as a compressed rod through a cooled nozzle whichopposes a conventional graphite electrode, wherein the extruded graphiteintercalation compound and the graphite electrode form a pair betweenwhich an arc is struck to rapidly heat the graphite intercalationcompound. In addition, a vacuum can be drawn to accelerate exfoliationand direct the extruded stream of exfoliated graphite intercalationcompound.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed.

The accompanying drawing is included to provide a further understandingof the disclosure, and is incorporated in and constitutes a part of thisspecification. The drawing illustrates an embodiment of the disclosure,and together with the description serves to explain the principles andoperations thereof.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially broken away side plan view of an apparatus for useas a reaction zone having a gas plasma therein and graphiteintercalation compounds passing therethrough to form nano-scalestructures.

DETAILED DESCRIPTION

The graphite flakes employed in the present disclosure comprisenaturally occurring graphite flakes. Natural graphite is a soft mineral,and possesses a Mohs hardness of 1 to 2, and exhibits perfect basalcleavage. While natural graphite occurs in deposits in differentcountries around the world in different forms, the preferred naturalgraphite is crystalline flake graphite, since other types, such asamorphous graphite and so-called “lump” graphite, are consideredundesirable for intercalation and exfoliation. Though so-calledmicrocrystalline graphite is not conventionally used in the preparationof exfoliated graphite, it is useful in the process of the presentdisclosure. Microcrystalline graphite, as is familiar to the skilledartisan, refers to graphite having a microcrystalline structure whichcan only be observed using x-ray diffraction techniques. While notnormally useful in making exfoliated and compressed graphite, a productwhere graphene layer dimensions are directly related to the usefulproperties of the product, for nano-structures the layer size ofmicrocrystalline graphite may represent a preferred starting material.

The graphite used in the process of the present disclosure should berelatively free from impurities, meaning it should have a purity of atleast about 90%, more preferably at least about 95%. In addition, thesize of the graphite flake (by which is meant the diameter of the flakealong the α axis, which is the direction parallel to the plane of theflake, or the graphene layers) can be a parameter in achieving theproduction of nano-structures. Advantageously, the flake used has anaverage a axis diameter of less than about 100 microns. More preferably,the α axis diameter of the flake employed during the practice of theprocess of the present disclosure is less than about 60 microns, mostpreferably less than about 30 microns for products recognizable to thefield as “conventional” nanotubes. However, since a desired use ofnano-structures such as nano-tubes is adsorption of materials thereon,such as adsorption of hydrogen for hydrogen storage for, e.g., protonexchange membrane fuel cell uses, the number of defect sites may be animportant factor since it is believed that adsorption takes place atdefect sites. Thus, it is likely that graphene layers of nano-scalethickness and millimeter-scale plane or length dimensions would containmany defects sites, both at its edges and within the plane of thestructure, and have many active sites for adsorption to occur, and wouldtherefore have advantageous uses in adsorption applications.

If desired, the graphite flakes can be annealed prior to intercalation,in order to increase the purity of the flakes and facilitateintercalation. Annealing involves exposing the raw graphite flakes tohigh temperatures, on the order of greater than about 2700° C. foranywhere from 15 minutes to one hour and more, as taught, for instance,by U.S. Pat. No. 6,982,874 to Smalc et al.

As noted above, Shane et al., in U.S. Pat. No. 3,404,061, describes acommon method for intercalating graphite flakes. Typically, naturalgraphite flakes are intercalated by dispersing the flakes in a solutioncontaining a mixture of nitric acid and sulfuric acid. The nitric acidand sulfuric acid components of the intercalant solution can be replacedby other acidic compounds, such as potassium chlorate, chromic acid,potassium permanganate, potassium chlorate, potassium dichromate,perchloric acid, or mixtures thereof. Most preferably, the intercalantsolution comprises components having a low boiling point and a low heatof vaporization, such as formic acid, acetic acid, or water, orcombinations thereof, so that most of the energy of exfoliation resultsin the greatest expansion of the GICs and, therefore, providing thegreatest possible force driving the graphene layers apart.

Intercalation can be so-called spontaneous intercalation, orelectrochemical oxidation of the graphite flakes can be practiced duringintercalation, as described in U.S. Pat. No. 6,406,612 to Greinke.

Other additives and process parameters can be employed to facilitateintercalation and expansion, such as the use of a supercritical fluid,such as supercritical carbon dioxide, as an intercalant, as described byKaschak et al. in International Publication No. WO 2004/108997.

As is familiar to the skilled artisan, a supercritical fluid is onewhich exhibits the properties of a gas when in the liquid state and theproperties of a liquid when in the gaseous state. When a gas such ascarbon dioxide is contained under high pressure and heated, it changesphysical properties, becoming a supercritical fluid. In this state, ithas the solvating power of a liquid and the diffusivity of a gas. Inshort, it has properties of both a gas and a liquid. This means thatsupercritical fluids work extremely well as a processing media for awide variety of chemical extractions.

While intercalation with a supercritical fluid can be advantageous forachieving intercalation to the Stage I degree, treatment of the Stage Iintercalated flakes with a supercritical fluid like supercritical carbondioxide can also function to reduce the tendency of the flake to“de-intercalate” to a lower degree of intercalation, and thus a higherStage of intercalation level (such as from Stage I to Stage V). Inaddition, treatment of the intercalated flake with a supercritical fluidafter completion of intercalation can also improve the expansion of theflake when heated.

While washing of the intercalated flake is commonly practiced whensheets of compressed particles of exfoliated graphite are beingprepared, washing tends to lower the degree of intercalation of theflake, thus resulting in a flake having a higher Stage of intercalationthan prior to washing (going from Stage II to Stage VII, for instance).Since the process of the present disclosure requires expansion of StageI, II or III GICs, a washing step should be avoided. Rather, if it isdesired to remove surface chemicals form the flake which remain afterintercalation, drying processes such as centrifugal drying, freezedrying, filter pressing, or the like, can be practiced, to at leastpartially remove surface chemicals without having a significant negativeeffect on degree of intercalation.

Once the graphite flakes are intercalated, and, if desired, exposed to asupercritical fluid and/or dried, they are exfoliated. Exfoliationshould be effected by suddenly exposing the Stage I, II or IIIintercalated graphite flakes to high heat. By “suddenly” is meant thatthe flakes are brought from a temperature at which the selected GIC isstable to a temperature substantially above its decompositiontemperature within a period of less than about 1 second, more preferablyno more than about 0.5 second, and even less than about 0.1 second, toachieve the rapid exfoliation desired for complete separation of atleast a plurality of graphene layers. Hot contact exfoliation methods,where the flake is directed contacted by a heat source, are notpreferred since during hot contact exfoliation the first exfoliatedflakes tend to act as insulators and insulate the balance of the flakes(and, thereby inhibit exfoliation). Generating heat within the GIC, forexample using an arc, high frequency induction or microwave, etc. ismuch preferred. The extreme heat of a gas plasma due to temperature(thousands of degrees C.) and the turbulence which would displace theexfoliate is highly preferred. More preferably, the temperature ofexfoliation is at least about 1500° C., and in certain preferredembodiments, the temperature of exfoliation is between about 1600° C.and 2400° C. Since graphite begins to degrade at 2500° C., this is thepractical upper limit for exfoliation temperature.

During exfoliation, the intercalant inserted between the graphene layersof the graphite (preferably between each graphene layer, as in the caseof Stage I intercalation) rapidly vaporizes and literally “blows” thegraphene layers apart, with such force that at least some of thegraphene layers separate from the exfoliated flake, and formnano-structures.

Exfoliation can be accomplished in certain embodiments by feeding theStage I, II or III GICs into a reaction zone which includes a regionwhere the temperature is at least about 2600° C.; of course, this isprovided that the graphite intercalation compound does not reach atemperature of greater than 2500° C. In certain embodiments, thereaction zone is at an average temperature of at least about 1500° C.This can be accomplished by feeding the GICs through a reaction zonehaving an inert gas plasma therein, or directly into an arc, to providethe high temperature environment needed for greatest expansion.Desirably, exfoliation occurs in a reducing gas environment, such ashydrogen, to adsorb the reducing gas onto active sites on the graphenelayers to protect the active sites from contamination during subsequenthandling. In addition, in some embodiments, the exfoliate can passdirectly into a liquid suitable to the end use of the exfoliate so thatany defect site present can accommodate the chemistry of the end use.

One advantageous method for exfoliation of the GICs prepared inaccordance with the present disclosure is to continuously extrude theintercalated flake as a rod through a cooled nozzle opposing a graphiteelectrode. The extruded flake and graphite electrode can form a pairbetween which an arc can be struck, which would rapidly heat the StageI, II or III GICs. A vacuum can be drawn on the system to accelerateexfoliation and direct the stream of exfoliated flake (and individualgraphene layers).

The individual graphene layers can then be collected by conventionalmeans, such as by centrifugal collectors, and the like. Contrariwise,the stream of exfoliated/exfoliating GICs as described above can bedirected at a suitable support for collection of the individualizedgraphene layers.

It is anticipated that many of the individual graphene layers, as theyseparate from the exfoliated flake, or sometime thereafter, willspontaneously assume a three-dimensional shape, such as a buckyball ornano-tube, while the remainder remain as flat plates. In either case,the separation of individual graphene layers from the GICs during orimmediately after exfoliation results in the production ofnano-structures. These nano-structures can be produced in large,commercially-significant volumes, and more cost efficiently thanconvention nano-structure production processes.

Referring now to FIG. 1., an apparatus 10 which can be used in anembodiment of the disclosure is illustrated. Apparatus 10 is a conduithaving a gas plasma 20 extending from electrode 22 therein; the gasplasma is at a temperature between about 2600° C. and about 10,000° C.The inner spaces of apparatus 10 are a reaction zone 30, in whichgraphite intercalation compounds 100 are exposed to a temperaturebetween about 1600° C. and about 2400° C. to form nano-scale structures110. Graphite intercalation compounds 100 are fed into reaction zone 30by feeder 32, where they are exposed to the temperature of exfoliation,form nano-scale structures 110, and exit reaction zone 30 through egress34, or other like conduit.

All patents and patent applications referred to herein are herebyincorporated by reference.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present disclosurewithout departing from the spirit and scope of the disclosure. It isintended that all such modifications and variations are part of thepresent disclosure provided they come within the scope of the foregoingclaims, and their equivalents.

1. A process for the production of nano-structures, comprising providinga graphite flake comprising graphene layers; intercalating the graphiteflake to form a graphite intercalation compound exhibiting Stage I, IIor III intercalation; and exfoliating the graphite intercalationcompound by exposing it to a temperature between about 1600° C. andabout 2400° C. such that a plurality of individual graphene layers areseparated from the graphite intercalation compound.
 2. The process ofclaim 1, wherein the graphite flake is intercalated with an intercalantcomprising formic acid, acetic acid, water, or combinations thereof. 3.The process of claim 1, wherein the graphite intercalation compound isexposed to a supercritical fluid prior to exfoliation.
 4. The process ofclaim 1, wherein the graphite intercalation compound is brought from atemperature at which it is stable to a temperature between 1600° C. and2400° C. within a period of no more than about 1 second.
 5. The processof claim 4, wherein the graphite intercalation compound is brought froma temperature at which it is stable to a temperature between 1600° C.and 2400° C. within a period of no more than about 0.5 second.
 6. Theprocess of claim 1, wherein exfoliation is accomplished by feeding thegraphite intercalation compound into a reaction zone which includes aregion where the temperature is at least about 2600° C.
 7. The processof claim 6, wherein exfoliation occurs in a reducing gas environment. 8.The process of claim 7, wherein exfoliation occurs in a hydrogenenvironment.
 9. The process of claim 8, wherein a vacuum is drawn toaccelerate exfoliation and direct the extruded stream of exfoliatedgraphite intercalation compound.
 10. The process of claim 1, wherein atleast some of the plurality of individual graphene layers spontaneouslyform a nano-tube or a Buckminster fullerene.